The purpose of the proposed research is to investigate the prelinguistic development and very early language development of children with Down syndrome. The research will focus on the development of babbling, gestures, and sensorimotor morphemes during the prelinguistic period and on lexical development, the major component of initial language acquisition, during the linguistic period. The research is designed to begin to delineate the onset and development of these important prelinguistic and early linguistic phenomena. There are four main objectives. The first is to describe the prelinguistic and early language development of children with Down syndrome, in comparison to normally developing children. The second is to describe the linguistic input provide by the mothers. The third is to describe aspects of mother-child interaction that potentially could have a major impact on early language development. The final objective is to address the potential effect of the factors studied other prelinguistic and lexical development of children with Down syndrome. The proposed project is a longitudinal study of 12 mother-infant with Down syndrome dyads and 12 matched mother-normally developing infant dyads. We will begin the study when the infants are 5 months old, before the onset of babbling. Dyads involving infants with Down syndrome will be followed for 31 months; dyads involve normally developing infants will participate for 19 moths, by which time all children should have begun to produce language referentially. Tape recordings of mother-infant dyads in a play situation, with a standard set of toys, will be made every 6 weeks. In addition, the children's intellectual development and receptive and productive language development will be tested at regular intervals. Data obtained from the play sessions and testing sessions will be used to address the objectives stated above. The results of this study will provide a solid basis for developing theories concerning the roles of child, mother, and mother-child interaction in determining the course of the child's prelinguistic and early lexical development. Importantly, the results also will provide a framework for delineating guidelines for intervention programs designed to enhance mother-child interaction relevant to prelinguistic and early linguistic development.